1. Field of the Invention
The invention generally relates to wireless communication systems and methods thereof, and more particularly, to methods for wireless communication for reducing the quantization effect of precoding operations utilizing finite codebooks in multiple-input multiple-output (MIMO) or multiple-input single-output (MISO) systems from a transmitter to a receiver.
2. Description of the Related Art
In wireless communications, multiple-input and multiple-output (MIMO) technology involves the use of multiple antennas at both the transmitter and receiver sides to improve communication performance. MIMO technology offers significant increases in data throughput and link ranges without additional bandwidth or transmit power via higher spectral efficiency and link reliability or diversity. One of the common functions in MIMO technology is precoding, which is a signal processing technique for MIMO communications to exploit the knowledge of the information of the downlink channel quality at the transmitter side. With precoding, the transmitter can transform signals using appropriate precoding matrices and perform power allocation adaptively according to the variation of link quality. It has been shown that precoding can substantially improve the spectral efficiency for MIMO communications.
Precoding is an effective technique improving the performance of MIMO-OFDM systems. In practical systems, the precoding matrices are pre-determined and only the index of the selected matrix is fed back.
Depending on the adopted criteria for assessing link performance, different objective functions exist for the computation of the optimal precoding matrix. For example, the minimum square error (MSE) and the achieved link capacity are two commonly adopted performance measures.
For the operation of precoding in MIMO communication systems, the information of the downlink channel quality generally has to be fed back from the receiver to the transmitter side. In general, precoding may require knowledge of channel state information (CSI) at the transmitter side. To achieve the optimal performance, the full information of the downlink channel quality or the optimal precoding matrix computed by the receiver has to be fed back to the transmitter side. Such signaling process incurs considerable overhead which scales with the number of antennas, i.e., the MIMO mode, and is undesirable in most cases. In current MIMO systems, the approach of codebook-based precoding is adopted to reduce feedback overhead. In codebook-based precoding, a number of selected precoding matrices are known to both the transmitter side and receiver side prior to the transmissions. The receiver simply feeds back the index of the most preferred precoding matrix which yields the best performance based on the measured link quality and the objective function to the transmitter via the feedback channel. For example, if the number of precoding matrices is set to 2n, the length of the feedback can be n bits. Codebook-based precoding can substantially reduce the feedback overhead.
Ideally, the size of the codebook should be made as small as possible to reduce receiver complexity and the feedback overhead. However, the link performance is degraded with a small codebook size as the quantization error for the optimal precoding matrix increases in this case. Precoding gain in MIMO systems can be increased by increasing the size of the codebook. However, at the same time the complexity of codeword selection and memory requirements increase exponentially with the number of bits. A tradeoff thus exists when designing the precoder codebook. The approach of utilizing multiple finite-sized codebooks has been proposed. In the multiple-codebook approach, the receiver may feed back the indices of the preferred precoding matrices for each of the adopted codebooks. The transmitter may then construct the precoding matrix by multiplying the precoding matrices indicated by the receiver. However, such multiple-codebook approaches involve the effort of designing and storing new codebooks at both the transmitter and the receiver sides, and still suffer from inevitable quantization errors as the possible choices of the precoders are limited by the product of the respective codebook sizes. Furthermore, the consequence of feedback error corresponding to the codebooks of relatively long feedback periods could severely degrade system performance (e.g., propagation error).